The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Oxygen concentration in an intake manifold is an important variable for combustion performance and emissions for diesel and lean-burn engines in general. For lean burn engines with an external EGR mechanism, EGR flow also brings unused oxygen (or fresh air charge) back into the intake manifold through the EGR valve since not all the oxygen in the cylinder is used during combustion. A mass air flow (MAF) sensor may provide a feedback signal used to control in-cylinder EGR amount. Controlling the fresh air flow is an indirect way of controlling the in-cylinder EGR amount. However, such technique does not take into account the variability of unused oxygen coming to the intake manifold through the EGR valve and the dynamical oxygen contributions from throttle and EGR will be different at the mixing point (intake manifold) during engine transients, which will have an adverse impact on the transient EGR control. Similarly, using an estimated EGR flow rate alone to control the in-cylinder EGR amount is not going to be effective during transients because the dynamic responses of flows through the throttle and through the EGR valve will be different. On the other hand, the oxygen concentration in the intake manifold captures the end effects of the distinct flows and is, therefore, directly related to the in-cylinder EGR amount and NOx emissions. To this end, an engine controller requires an accurate estimate of the oxygen concentration in the intake manifold to robustly control the in-cylinder mixture and in turn combustion.